This invention relates to a joint prosthesis.
More particularly, the invention relates to a prosthesis which can be used to replace a metacarpo phalangeal joint although the principles of the invention are applicable to prostheses for other joints where there is some degree of biaxial rotation required.
The metacarpo phalangeal joint prosthesis is employed by surgeons to help patients with arthritis affecting an MCP joint. The aim of the prosthesis is to correct deformity, relieve pain, improve the range of movement of the joint, maintain stability, and therefore improve hand function. Known MCP joint prosthesis include single piece silastic products known as the Swanson and Sutter joints. In these products, the essential pivotal action is provided by an integral web hinge between the metacarpal and phalangeal components of the prosthesis. These components include integral intramedullary stems which are not fixed in place but are slidably retained in the respective bone cavities. The stems are rectangular to prevent rotation and are designed to piston within the cavity. The products rely to an extent on the development of scar tissue to encapsulate the joint.
Prosthesis of this type have been successful in achieving pain relief. The arc of motion achieved, around 40xc2x0 to 60xc2x0, is sufficient to improve hand function relative to pre-insertion arthritic conditions. On the other hand, it is considered by many that the prosthesis does not provide adequate stability for the joint and there is a significant incidence of fracture at the hinge and/or stems, and of dislocation. The latter occurs when the stems slide out of the bone cavities after significant wear. There has also been an increasing longer term association with silicon synovitis, and with disintegration of the prosthesis.
An adaptation of the integral hinge prosthesis was the Niebauer prosthesis in which the stems were surrounded by fibre mesh so that intramedullary fixation could be obtained by fibrous growth into the mesh. Clinical review demonstrated that flexion at the hinge did not occur due to soft tissue interposition, and that there was a tendency for fracture at the hinge and at the distal stem. The Helat MCP flap joint had a dorsal ulnar based flap to maintain central position with respect to the extensor tendon.
In general, integral hinge prostheses have been characterised, relative to earlier highly constrained linked structures, by increased restriction of movement imposed by the design of the prosthesis, but by a corresponding increased reliance on soft tissues around the joint to provide stability, maintain alignment and correction of deformity. Problems met have included implant failure due to material failure or fracture at the hinge or stem, inadequate range of movement due to soft tissue interposition at the hinge, and inadequate rigidity of the implant leading to recurrence of ulnar deviation deformity.
In an attempt to reduce the stresses on the implant, and to better stimulate the normal anatomy of the articulating surfaces at the MCP joint, unrestrained implants have been proposed. An example was the WEL MCP joint disclosed in U.S. Pat. No. 4,242,759 in which the metacarpo and phalangeal components are not connected together. In the second embodiment described in that patent, the phalangeal component has a transverse ridge which traverses a complementary trough or groove in the metacarpo component. The different lateral articulation of the joint at the extended and clench positions is simulated by providing a close fit between the ridge and groove at one position and a fit with lateral articulating freedom at another position. A published paper concluded that this jointed tended to dislocate as it was too reliant on the stability of surrounding tissue, a condition which cannot be met in cases of rheumatoid arthritis. A further proposal in this category in the Stokoe-Unsworth prosthesis, described in PhD thesis by Stokoe, which is still at a developmental stage. This prosthesis is an unconstrained surfacing implant with a spherical contoured metacarpo component and a corresponding saucer-shaped phalangeal component.
U.S. Pat. No. 3,946,445 discloses a structure somewhat similar to that of U.S. Pat. No. 4,242,759 save that the rib and an associated pair of grooves are on the metacarpo component. Like the other patent, this reference shows mounting of the components by respective tapered solid intramedullary stems.
U.S. Pat. No. 5,938,700 is another example of a metacarpo phalangeal joint. In this prosthesis, a ball and socket type of connection is provided, the ball being provided with a dovetail rib which is received within a dovetail slot formed on respective members. This is an example of a restrained implant and, because of the interlocking-dovetails, is essentially incapable of providing for any ulnar or radial deviation.
These unconstrained prostheses were designed in an attempt to reduce the stresses on the implant and at the site of fixation, and to recreate the normal anatomy of the articulating surfaces at the MCP joint. The success of these implants depends on early surgery, and on good soft tissue reconstruction. They are highly reliant on the soft tissue surrounding the joint for joint stability and the prevention of deformity recurrence. The difficulty with these premises is that early surgery is rarely needed or attempted since, at that stage, the patients have good hand function and surgery is generally not therefore indicated. Moreover, in the rheumatoid process where soft tissue is inflamed and where there is a tendency for soft tissue deformation to occur, it is unlikely that unrestrained prosthesis will stand the test of time.
The current practical situation, therefore, is that MCP prostheses are typically only recommended in extreme cases. Surgeons generally do not regard presently available products as entirely satisfactory. The technology is relatively crude and unsatisfactory in comparison to knee and hip joints, which have reached a high level of reliability and are therefore commonly inserted. There is a need for an MCP prosthesis that allows the joint a functional range of movement in most of the normal planes of joint motion while at the same time incorporating some constraint to limit soft tissue deformity and to provide stability. It is an object of the invention to at least in part meet this objective.
According to the present invention there is provided a prosthesis for replacing a joint between first and second articulating bones the prosthesis comprising first and second joint members and wherein:
the first joint member includes a first mounting member which in use is mounted in an intramedullary canal of the first bone;
the second joint member includes a second mounting member which in use is mounted in an intramedullary canal of the second bone;
the first joint member has a ball having a first part spherical surface and a slot extending transversely through the ball;
the second joint member has a socket having a second part spherical surface which is complementary to first spherical surface and a guide peg which projects from the second spherical surface and in use is located in said slot; and
wherein the guide peg is loose fit within the slot thereby permitting biaxial rotation of the second member relative to the first member about at least first and second axes.
The invention also provides a prosthesis for replacing a joint between first and second articulating bones the prosthesis comprising first and second joint members and wherein:
the first joint member includes a first mounting member which in use is mounted in an intramedullary canal of the first bone;
the second joint member includes a second mounting member which in use is mounted in an intramedullary canal of the second bone;
the first joint member has a ball having a first part spherical surface and a slot extending transversely through the ball;
the second joint member has a socket having a second part spherical surface which is complementary first spherical surface and a guide peg which projects from the second spherical surface and in use is located in said slot; and
wherein the socket is encircled by an annular lip.
The invention also provides a method of providing a prosthesis for a joint between first and second articulating bones using a prosthesis as defined above including the steps of removing adjacent parts of the first and second bones to expose first and second intramedullary canals and fixing the first and second mounting members in the first and second intramedullary canals, respectively.
In the preferred form of the invention, the first and second bones are metacarpal and phalangeal bones, respectively.
Preferably further, the method enables the prosthesis to provide up to about 90xc2x0 of flexion, 10xc2x0 of extension, 25xc2x0 radial deviation and/or 5xc2x0 ulnar deviation and a small degree of axial rotation, i.e. pronation and/or supination.
The invention will now be further described with reference to the accompanying drawings, in which:
FIG. 1 is a schematic view of the bones of a right hand showing a prosthesis of the invention mounted therein;
FIG. 1A is an enlarged schematic view of some of the bones;
FIG. 2 is a side view of the ball component of the prosthesis with the dorsal side on the right;
FIG. 3 is a plan view of the ball component as viewed from the dorsal side;
FIG. 4 is an end view of the ball component;
FIG. 5 is a side view of the socket component with the dorsal side on the left;
FIG. 6 is a plan view of the socket component;
FIG. 7 is a sectional view along the lines 7xe2x80x947;
FIG. 8 is an end view of the socket component;
FIG. 9 is a cross-sectional side view of an assembled prosthesis in a neutral position;
FIG. 9A is a cross-sectional underside plan view of an assembled prosthesis in a neutral position;
FIG. 10 is a schematic cross-sectional side view of an assembled prosthesis with 10xc2x0 extension;
FIG. 11 is a schematic cross-sectional side view of an assembled prosthesis with 80xc2x0 flexion;
FIG. 12 is a schematic plan view of the assembled prosthesis;
FIG. 13 is a schematic plan view of the prosthesis with 5xc2x0 ulnar deviation; and
FIG. 14 is a schematic plan view of the prosthesis with 25xc2x0 radial deviation.